Biomechanics is the study of the applications of mechanical engineering to the human body. Specifically, it is used to examine the failure strength of bodily tissues and the physical mechanisms required to cause certain injuries. For instance, an accident reconstructionist may calculate that a person would have experienced a blow to the head in the range of 30g as a result of striking the windshield in a frontal collision. The biomechanical engineer would use the direction and severity of this impact to determine whether this accident would be likely to cause a spinal or head injury.
We use our knowledge of physics and mechanical properties of the musculoskeletal system to give a detailed description of the expected effects of a particular impact on an occupant.
Injuries cannot occur if an injury mechanism is not present. The dynamics of the vehicle during an accident define an occupant's motion. It is the force and direction of this motion which produces potential injury mechanisms. Thus, a biomechanical analysis must be done in conjunction with the accident reconstruction. For example, during a rollover accident it is impossible to define an occupant's ejection point and corresponding injuries without first defining the rollover sequence. Likewise, it is impossible to ascertain whether a seatbelt could prevent specific injuries without first defining an occupant's motions.
Doctors and other medical professionals are trained to diagnose injuries and treat them. This does not require a detailed understanding of the mechanism of the actual injury. For this reason, doctors are not trained, or necessarily knowledgeable, in the amount and direction of force required to cause a specific injury. This sort of determination falls solely within the scope of bio-mechanics.